1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device. More precisely, it relates to an organic EL device suitable to light sources for displays, printer heads and others for domestic and industrial use.
2. Description of the Related Art
Reference 1, JP-A-4-297076 discloses one example of conventional organic EL devices. As in FIG. 10, the organic EL device 60 disclosed by Reference 1 comprises an organic laminate layer of three organic films 52, 54 and 56 as sandwiched between a cathode layer 58 and an anode layer 50 of a transparent electrode. Of the three organic films, the first organic film 52 that is adjacent to the cathode layer 58 is doped with a donor impurity, while the second organic film 54 that is adjacent to the anode layer 50 is doped with an acceptor impurity. As the acceptor impurity, used is any of CN-substituted compounds and quinone compounds (e.g., chloranil). The third organic film 56 sandwiched between the first organic film 52 and the second organic film 54 is a light emission layer. Carriers are confined in the light emission layer 56 by the adjacent first and second organic films 52 and 54. Accordingly, the organic EL device 60 could produce high luminance (that is, its luminous efficiency is high) even at low driving voltage.
Reference 2, xe2x80x9cDigest of Technical Papers, SID""97, p. 775, 1997xe2x80x9d discloses another example of conventional organic EL devices. In the organic EL device disclosed by Reference 2, the electron transportation layer is made of a material that comprises tris(8-hydroxyquinoline)Al (Alq complex) with Li added thereto.
In the organic EL device disclosed by Reference 1, CN-substituted compounds and quinone compounds used as the acceptor impurity have good electron transportation capabilities but their acceptor capabilities are strong. Concretely, their electron affinity is on the level of at least 3.7 eV and is high. Therefore, the acceptor impurity of those compounds often reacts with compounds that constitute the light emission region, thereby forming charge-transfer complexes or exciplexes. For these reasons, the organic EL device faces the problems of luminance depression and short lifetime.
In addition, in the organic EL device disclosed by Reference 1, the difference in the electron affinity between the donor impurity-doped, first organic layer and the light emission region is at least 0.5 eV and is large. Therefore, in this, a blocking contact is formed at the interface between the first organic layer and the light emission region. In that condition, the electron injection from the first organic layer to the light emission region is often unsatisfactory, and, as a result, the organic EL device shall face an additional problem of further reduction in its luminous efficiency.
On the other hand, Alq complexes in the organic EL device disclosed by Reference 2 also contain nitrogen atoms. In this, the electron transportation layer that comprises such an Alq complex and a Li compound has good electron transportation capabilities, but easily forms charge-transfer complexes or exciplexes. In addition, the organic EL device often requires high driving voltage. Therefore, like that disclosed by Reference 1, the organic EL device disclosed by Reference 1 also faces the problems of luminance depression and short lifetime.
Given that situation, we, the present inventors assiduously studied the problems noted above, and, as a result, have found that, when a nitrogen-free aromatic compound is used in the electron injection region, or when a nitrogen-containing aromatic compound, if used in that region, is combined with a specific reducing dopant, then the driving voltage for the organic EL device can be reduced and the luminance of the device can be increased and, in addition, the lifetime of the device can be prolonged. Based on these findings, we have completed the present invention. Specifically, the object of the invention is to provide an organic EL device that produces high luminance even at low driving voltage and has a long lifetime.
According to the first aspect of the invention, there is provided an organic EL device comprising at least an anode layer, a light emission region, an electron injection region and a cathode layer as laminated in that order, wherein the electron injection region contains a nitrogen-free aromatic compound and a reducing dopant and the electron affinity of the electron injection region is controlled to fall between 1.8 and 3.6 eV.
As containing a nitrogen-free aromatic compound, the electron injection region has good electron injection capabilities, and, in addition, this is prevented from reacting with the material that constitutes the light emission region adjacent thereto. Specifically, the nitrogen-free aromatic is composed of carbon and hydrogen, or of carbon, hydrogen and oxygen, and does not have any nitrogen-containing groups such as nitrogen-containing aromatic groups and electron-attractive groups (e.g., xe2x80x94CN, xe2x80x94NO2, amido, imido). Therefore, in the constitution of the organic EL device of the invention, charge-transfer complexes or exciplexes with low luminous efficiency are well prevented from being formed in the interface between the electron injection region and the light emission region.
In addition, since the electron injection region contains a reducing dopant along with a nitrogen-free aromatic compound, the aromatic skeleton of the nitrogen-free aromatic compound in the region is well reduced to be in an anionic state. Accordingly, in the organic EL device of the invention, charge-transfer complexes or exciplexes with low luminous efficiency are much more prevented from being formed, whereby the luminance of the device is increased and the lifetime thereof is prolonged.
Moreover, since its electron affinity is suitably controlled, the electron injection region shall have more improved electron injection capabilities, and, in addition, charge-transfer complexes or exciplexes are well prevented from being formed at the interface between the electron injection region and the light emission region. Further, the blocking contact is prevented from being formed at the interface between the electron injection region and the light emission region. With that constitution, therefore, the luminance of the device is increased and the lifetime thereof is prolonged.
In the organic EL device of the first aspect of the invention, it is desirable that the electron injection region has a glass transition point of not lower than 100xc2x0 C.
The organic EL device in which the electron injection region has a glass transition point of not lower than 100xc2x0 C. could have high heat resistance. For example, the device is resistant to heat at a temperature of 85xc2x0 C. or higher. In the device with that constitution, the electron injection region is protected from being broken within a short period of time by the Joule s heat to be generated through current injection from the current injection layer to the light emission region while the device is driven to emit light. As a result, the device could have a prolonged lifetime.
In the organic EL device of the first aspect of the invention, it is also desirable that the aromatic compound contains a residue of at least one aromatic ring selected from the group consisting of anthracene, fluorene, perylene, pyrene, phenanthrene, chrysene, tetracene, rubrene, terphenylene, quaterphenylene, sexiphenylene and triphenylene.
In the organic EL device of the first aspect of the invention, it is still desirable that the aromatic compound contains a residue of at least one aromatic ring selected from the group consisting of styryl-substituted aromatic rings, distyryl-substituted aromatic rings and tris-styryl-substituted aromatic rings.
In the organic EL device of the first aspect of the invention, it is still desirable that the reducing dopant is at least one substance selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals, alkali metal oxides, alkali metal halides, alkaline earth metal oxides, alkaline earth metal halides, rare earth metal oxides and rare earth metal halides.
In the organic EL device of the first aspect of the invention, it is still desirable that the reducing dopant has a work function of at most 3.0 eV.
In the device, the reducing dopant having such a specifically defined work function could satisfactorily exhibit its reducing ability. With the reducing dopant, therefore, the driving voltage for the device can be reduced, and, in addition, the luminance of the device can be increased and the lifetime thereof can be prolonged.
In the organic EL device of the first aspect of the invention, it is still desirable that the reducing dopant is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs.
The reducing dopant of that type has especially high reducing ability. Therefore, even though a relatively small amount of the reducing dopant is added thereto, the device could produce much increased luminance of, for example, a high value of at least 500 cd/m2 (when driven at 7 V), and, in addition, its lifetime is much prolonged. For example, the device has a half lifetime of 1000 hours or longer.
In the organic EL device of the first aspect of the invention, it is still desirable that the energy gap in the electron injection region is at least 2.7 eV.
Having such a large value of energy gap, the electron injection region is well protected from hole transfer thereinto. In that condition, therefore, the electron injection region itself is prevented from emitting light.
In the organic EL device of the first aspect of the invention, it is still desirable that the ratio of the aromatic compound to the reducing dopant in the electron injection region falls between 1:20 and 20:1 (by mol).
If the ratio of the aromatic compound to the reducing dopant in the electron injection region oversteps the defined range, the luminance of the organic EL device will lower and the lifetime thereof will be shortened.
In the organic EL device of the first aspect of the invention, it is still desirable that both the light emission region and the electron injection region contain the same nitrogen-free aromatic compound.
Containing the same nitrogen-free aromatic compound, the adhesion of the two regions is good. As a result, electrons are smoothly transferred from the electron injection region to the light emission region, and, in addition, the mechanical strength of the device is increased.
In the organic EL device of the first aspect of the invention and also in that of the second aspect of the invention to be mentioned hereinunder, it is still desirable to provide an interlayer between the cathode layer and the electron injection region and/or between the anode layer and the light emission region.
With such an interlayer, the luminance of the device and even the half lifetime thereof can be greatly increased and prolonged.
According to another aspect (the second aspect) of the invention, there is provided an organic EL device comprising at least an anode layer, a light emission region, an electron injection region and a cathode layer as laminated in that order, wherein the electron injection region contains an electron-transporting compound and a reducing dopant having a work function of at most 2.9 eV, and the electron affinity of the electron injection region is controlled to fall between 1.8 and 3.6 eV.
In the device in which the electron injection region contains a reducing dopant with a specifically defined work function, even though the electron-transporting compound in that region is oxidized, the compound could be effectively reduced to be in an anionic state. Therefore, in the constitution of the organic EL device, charge-transfer complexes or exciplexes are well prevented from being formed. Accordingly, the driving voltage for the device can be reduced, the luminance of the device can be increased, and the lifetime thereof can be prolonged. Specifically, in the device of the second aspect, the reducing ability of the reducing dopant is high. Therefore, even when the electron-transporting compound in the device of the second aspect contains a nitrogen-containing group (or nitrogen atom) such as a nitrogen-containing aromatic ring or an electron-attractive group, being different from that in the device of the first aspect, the device of the second aspect still has the advantage of retarding the reaction of the electron-transporting compound with the material that constitutes the light emission region.
Moreover, since the electron affinity of the electron injection region is suitably controlled, charge-transfer complexes or exciplexes are well prevented from being formed in the interface between the electron injection region and the light emission region, and, in addition, the blocking contact is prevented from being formed at the interface between the electron injection region and the light emission region. With that constitution, therefore, the luminance of the device is increased and the lifetime thereof is prolonged.
In the organic EL device of the second aspect of the invention, it is desirable that the reducing dopant is at least one alkali metal or alkaline earth metal selected from the group consisting of Na, K, Rb, Cs, Ca, Sr and Ba.
The reducing dopant of that type has especially high reducing ability. Therefore, even though a relatively small amount of the reducing dopant is added thereto, the device could produce much increased luminance of, for example, a high value of at least 500 cd/m2 (when driven at 7 V), and, in addition, its lifetime is much prolonged. For example, the device has a half lifetime of 1000 hours or longer.
In the organic EL device of the second aspect of the invention, it is still desirable that the electron-transporting compound includes nitrogen-containing heterocyclic compounds.
Nitrogen-containing heterocyclic compounds have good electron-transporting ability. With that constitution, therefore, the device has the advantage of much higher electron injection capabilities.
In the organic EL device of the second aspect of the invention, it is still desirable that the nitrogen-containing heterocyclic compound is at least one selected from the group consisting of nitrogen-containing complexes, quinoxaline derivatives, quinoline derivatives, oxadiazole derivatives, thiadiazole derivatives and triazole derivatives.
In the organic EL device of the second aspect of the invention, it is still desirable that the ratio of the electron-transporting compound to the reducing dopant in the electron injection region falls between 1:20 and 20:1 (by mol).
If the ratio of the electron-transporting compound to the reducing dopant in the electron injection region oversteps the defined range, the luminance of the organic EL device will lower and the lifetime thereof will be shortened.
In the organic EL device of the second aspect of the invention, it is still desirable that the electron injection region has a glass transition point of not lower than 100xc2x0 C.
With the electron injection region having such a specifically defined glass transition point, the heat resistance of the device is improved and the lifetime thereof is prolonged.
In the organic EL device of the second aspect of the invention, it is still desirable that both the light emission region and the electron injection region contain the same electron-transporting compound.
Containing the same electron-transporting compound, the adhesion of the two regions is good. As a result, electrons are smoothly transferred from the electron injection region to the light emission region, and, in addition, the mechanical strength of the device is increased.